SPECTRA AND STRUCTURE OF SMALL-RING COMPOUNDS .62. RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT OF AB-INITIO CALCULATIONS OF CYCLOPROPYLMETHYLTRIFLUOROSILANE
Ts. Little et al., SPECTRA AND STRUCTURE OF SMALL-RING COMPOUNDS .62. RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT OF AB-INITIO CALCULATIONS OF CYCLOPROPYLMETHYLTRIFLUOROSILANE, Journal of Raman spectroscopy, 25(9), 1994, pp. 735-746
The Raman (3500-20 cm-1) and infrared (3500-50 cm-1) spectra of gaseou
s and solid cyclopropylmethyltrifluorosilane, c-C3H5CH2SiF3, were reco
rded. Additionally, the Raman spectrum of the liquid was obtained alon
g with qualitative depolarization values. The spectra of the gas and l
iquid were interpreted on the basis of the predominance of a conformat
ion having a gauche structure and this form exists exclusively in the
solid. Spectral evidence to support the existence of a higher energy c
is conformer where the SiF3 group is cis to the cyclopropyl ring is ob
served in the Raman spectra of the gas and liquid. From studies of the
Raman spectra at various temperatures, the conformational enthalpy di
fference was determined to be 490 +/- 122 cm-1 (1.4 +/- 0.35 kcal mol-
1) and 548 +/- 110 cm-1 (1.57 +/- 0.32 kcal mol-1) (1 kcal = 4.184 kJ)
for the gas and liquid, respectively. The cyclopropyl torsion was obs
erved as a weak infrared band centered at 77 cm-1 in the spectrum of t
he gas. A complete assignment of the normal vibrational modes is provi
ded. Ab initio calculations utilizing the RHF/6-31G and MP2/6-31G* ba
sis sets are consistent with the gauche rotamer lying 554 cm-1 (1.6 kc
al mol-1) and 296 cm-1 (0.85 kcal mol-1), respectively, lower in energ
y than the cis conformation. It is estimated that there is only 4-10%
of the higher energy conformer present in the vapor at ambient tempera
ture. Optimized structural parameters were obtained for both conformer
s with both basis sets and the results should provide good starting pa
rameters for the analysis of the electron diffraction data. The force
field obtained with the RHF/6-31G basis set was used to perform a nor
mal coordinate analysis. These results are compared with corresponding
quantities for some similar molecules.